A semiconductor substrate, particularly a silicon single crystal wafer (hereinafter referred to occasionally as a “substrate”), is used as the substrate for the manufacture of a highly integrated MOS device. Most silicon single crystal wafers are substrates that are cut from an ingot of silicon single crystal produced by the Czochralski (CZ) method.
The silicon single crystal wafer of this kind suffers supersaturated presence of oxygen that has been incorporated during the production of a single crystal. This oxygen is precipitated during the subsequent device processing and eventually causes an oxygen precipitate to form in the substrate. When this oxygen precipitate is present in a significant amount in the substrate, the heavy metals subsequently incorporated during the device process are believed to be absorbed in the substrate, and this leads to the effect that the surface of the substrate (i.e. a device active layer) is kept clean.
This effect can be used for intrinsic gettering (IG), which is used for the purpose of preventing deterioration of device properties by heavy metals contamination. The silicon single crystal substrate, therefore, is desired to induce oxygen precipitation moderately during the course of device processing.
For the purpose of securing the gettering ability, the silicon single crystal wafer requires presence of oxygen precipitate beyond a fixed density at the center of the thickness thereof. As a result of a test conducted to date, it has been ascertained that when the silicon single crystal wafer secures presence of oxygen precipitate at a density of not less than 1×109 precipitates/cm3 midway between the surfaces of the wafer, it manifests a gettering ability against such heavy metals as Fe, Ni, and Cu even in a heat treatment performed in low-temperature device processing wherein the highest temperature is equal to or less than 1100° C.
Meanwhile, the silicon semiconductor substrate with a silicon single crystal layer (epi-layer) deposited (epi-deposited) on the surface of a silicon single crystal wafer (the so-called epitaxial wafer) has found acceptance as a substrate of still higher quality. Numerous epitaxial wafers have been used for manufacturing devices of higher density and higher integration.
In the present disclosure, the silicon single crystal wafer lacking an epi-deposition is called a mirror wafer and distinguished from a wafer with epi-deposition. The substrate on which the epi-deposition is effected is referred to herein as a “base plate.”
The epitaxial wafer allows no presence of such grown-in defects as COP (crystal originated particle) on the surface of the base plate and is known to enhance such device property as a function of oxide film pressure resistance.
The epitaxial wafer is typically produced by a process of epitaxial deposition that consists in depositing a silicon single crystal layer at a high temperature of at least 1100° C. Thus, the epitaxial wafer subjected to this high-temperature treatment is less capable of inducing oxygen precipitation during device processing and becomes inferior in the gettering property to the aforementioned mirror wafer that lacks epi-deposition. The cause of this disadvantage is believed to be that the oxygen precipitation nuclei are diminished or eliminated during the high-temperature heat treatment of the epi-deposition process and therefore oxygen precipitation does not occur in the subsequent device processing.
As one way of compensating for the shortage of oxygen precipitation in the epitaxial wafer, a method that comprises subjecting the substrate, prior to undergoing epi-deposition, to a heat treatment and subsequently carrying out the epi-deposition has been proposed. Since this method entails addition to the number of steps of process, it may lead to the disadvantage of increasing the cost of production of the epitaxial wafer.
By contrast, methods have been proposed for producing an epitaxial wafer that induces oxygen precipitation during device processing without requiring a heat treatment in advance of epi-deposition. One method to produce an epitaxial wafer uses a carbon-added base plate as a substrate (refer to Patent Document 1). Other methods for producing an epitaxial wafer use a nitrogen-added base plate as a substrate (refer to Patent Document 2 and Patent Document 3) Another method for producing an epitaxial wafer uses as a substrate a base plate having both carbon and nitrogen added (refer to Patent Document 4).
In the case of the epitaxial wafer resulting from subjecting a carbon-doped substrate to epi-deposition, oxygen precipitation does not occur at temperatures above 800° C., though it does occur in a temperature range falling short of 800° C. Device processing rarely involves a heat treatment below 800° C., and therefore, possibly reveals deficiency in the IG ability because of the failure to induce oxygen precipitation sufficiently.
By contrast, in the case of the epitaxial wafer resulting from subjecting a nitrogen-doped substrate to epi-deposition, the oxygen precipitation occurs even in a temperature range exceeding 800° C. and, what is more, the density of the oxygen precipitate is always constant irrespective of the conditions of heat treatment. Thus, it is made possible to produce an epitaxial wafer that is capable of manifesting the IG ability in any device process.
This advantage may be due to the fact that the addition of nitrogen results in forming thermally stable nuclei of oxygen precipitation during the growth of crystal. These nuclei are not diminished during the process of epi-deposition and, as a result, oxygen precipitate occurs based on these nuclei of oxygen precipitation during the device heat treatment subsequent to the epi-deposition. Since the thermal hysteresis during crystal growth serves as some kind of pretreatment for epi-deposition, it may be inferred that the base plate immediately subsequent to the growth of crystal is already in a state capable of inducing oxygen precipitation even after the epi-deposition. Using this base plate as the substrate secures the oxygen precipitation subsequent to the epi-deposition without the heat treatment prior to the epi-deposition. What is more, since the density of oxygen precipitation in the epitaxial wafer produced from the nitrogen-added base plate is constant irrespective of heat treatment, it is possible to produce an epitaxial wafer with IG ability for any device processing. Thus nitrogen is believed to be superior as a dopant for this purpose as compared to other elements such as, for example, carbon.
Thus, the nitrogen-doped base plate serves to secure stable oxygen precipitation without requiring an extra heat treatment. It has been found, however, that when the nitrogen-doped base plate is subjected to epi-deposition, the resultant epi-layer suffers occurrence of such crystal defects as N-SF and E-pit.
FIGS. 1a, b, and c are explanatory drawings illustrating the N-SF defect among other crystal defects. An epitaxial wafer 100 illustrated in the drawing is a product of deposition of an epi-layer 102 on a substrate 101 adapted for epi-deposition. Incidentally, FIG. 1a is a schematic perspective view of the inner structure of the epitaxial wafer 100, FIG. 1b is a plan view of the N-SF part as observed from above, and FIG. 1c is a cross section of the N-SF part.
The N-SF is an interstitial atom-type stacking fault on the {111} face extending from an interface 103 between the substrate 101 and the epi-layer 102 to the surface 104 of the epi-layer 102. Particularly when the stacking fault in the substrate 101 appears on the interface 103, the N-SF tends to occur with that fault 105 as the starting point.
The N-SF, when the substrate 101 is subjected to epi-deposition, is shaped as an equilateral triangle having a side length of about T×√2 [μm], wherein T [μm] denotes the thickness of epi-layer. Since the N-SF of this structure, when visually examined with a surface analyzer, appears as the same scattered image as a foreign matter on the base plate, the number of occurrences of the N-SF can be evaluated by subjecting the base plate, subsequent to the epi-deposition, to measurement with the surface analyzer.
FIGS. 2a, b, and c are explanatory drawings illustrating the E-pit defect among other crystal defects. An epitaxial wafer 100a illustrated in the drawing is a product of selectively etching an epitaxial wafer having generated E-pit. Incidentally, FIG. 2a is a schematic perspective view, FIG. 2b is a plan view of the E-pit part as observed from above, and FIG. 2c is a cross section of the E-pit part.
The E-pit consists of one or more dislocations 107 extending from the defect 105 present in the interface 103 between the substrate 101 and the epi-layer 102 to the surface of the epi-layer 102. Though the E-pit escapes detection with a surface analyzer, the number of E-pits can be evaluated by counting the pits which are formed by subjecting the surface of the base plate subsequent to the epi-layer deposition to such selective etching as light etching and Secco etching. The epitaxial wafer surface subsequent to selective etching is denoted as 100a. Incidentally, the etching amount [μm] by the selective etching should be equal to or smaller than the film thickness T [μm] of the epi-layer.
The N-SF and the E-pit are presumed to be defects that are formed in the epi-layer 102 from crystal defects that were present from the beginning in the substrate 101 as the starting points.
If the N-SF is present in an amount exceeding 0.05 occurrences/cm2 or the E-pit is present in an amount exceeding 0.05 occurrences/cm2, the probability that the defect will induce a failure in a device having an electrode surface area of 20 mm2, for example, will exceed 5%. Since an electrode including such occurrences of defects suffers deterioration of such electric properties as TDDB, a base plate in which such occurrences of defects are present cannot be used as a silicon semiconductor base plate for a high-quality device. It is, therefore, desired that the amounts of N-SF or E-pit be kept below 0.05 occurrences/cm2.
These peculiar crystal defects as N-SF and E-pit that are generated in the epi-layer 102 in consequence of the addition of nitrogen have a close relation with the defect region present in the nitrogen-added base plate prior to the epi-deposition. For the purpose of preventing the epi-layer defect, therefore, it is desired to control the defect region present in the base plate prior to the epi-deposition.
FIG. 3 is an explanatory drawing illustrating the relation of the defect region and the nitrogen concentration in the silicon single crystal that has been pulled by the Czochralski (CZ) method. FIG. 3a is a graph showing the relation between the defect region present in a base plate used for a substrate prior to epi-deposition and the nitrogen concentration and FIG. 3b is a schematic drawing showing the defect region in a silicon single crystal ingot 200 in the course of being pulled and the nitrogen concentration distribution.
The CZ method, as is generally known, consists in pulling a silicon single crystal ingot 200 from a silicon melt 201 upward and meanwhile causing it to grow. In a base plate cut out of this silicon single crystal ingot 200, three kinds of defect regions (V region, OSF region, and I region) are present as shown in FIG. 3a. 
First, the V region is a region into which excess vacancies are introduced from the solid-liquid interface during the growth of crystal. It suffers the presence of voids resulting from aggregation of such atomic vacancies.
The OSF region is a region into which excess vacancies are introduced from the solid-liquid interface during the growth of crystal. This region is where OSF occurs when the silicon single crystal wafer is subjected to an oxidizing heat treatment. The term “OSF” as used herein refers to a disk-like stacking fault measuring about several μm in diameter and including oxygen precipitates (OSF nuclei) at the center. It is formed in consequence of the phenomenon that the interstitial atoms generated by the oxidizing heat treatment from oxide film/silicon interface aggregate on the periphery of the OSF nucleus. The term “OSF nucleus” refers to a special oxygen precipitate possessing the nature of gathering interstitial atoms among other oxygen precipitates. It is presumed to be already present in the base plate at the stage immediately after the growth of crystal. Since the OSF nucleus has a small size (presumed to be not more than 10 nm), it cannot be detected by the existing method of evaluation using a contamination meter or an infrared tomography. The presence of OSF, therefore, is not ascertained unless the sample is subjected to the oxidizing heat treatment.
The I region is a region into which excess interstitial atoms are introduced from the solid-liquid interface during crystal growth. It includes a dislocation loop resulting from the aggregation of interstitial atoms.
From prior information, it has been known that the occurrence of the defect region in a base plate depends on the nitrogen concentration and the condition of V/G crystal growth, wherein V denotes the pulling speed [mm/min] and G denotes the average temperature gradient [° C./mm] from the melting point to 1350° C. in the direction of the axis of crystal growth (refer to non-Patent Document 1 and Non-Patent Document 2, which are hereby incorporated herein for all purposes).
In the case of a base plate cut from a silicon single crystal ingot with no nitrogen dopant, an increase of V/G beyond a specific value results in causing excess vacancies to occur and form a V region or an OSF region in the base plate. Then, a decrease of V/G below a specific value results in causing excess interstitial atoms to occur and form an I region in the base plate. Nitrogen meanwhile has an effect on the amounts of vacancies and interstitial atoms that occur via the solid-liquid interface. Thus, the defect region of a base plate cut from a nitrogen-doped silicon single crystal ingot can be expressed by a two-dimensional defect region map having nitrogen concentration and V/G as two axes as shown in FIG. 3a. 
Then, a single pull of single crystal out of the nitrogen-doped silicon, as shown in FIG. 3a, has a certain spread of nitrogen concentration and V/G values forming a rectangular region (called growth condition region) in the nitrogen concentration-V/G diagram. This is because the nitrogen-doped silicon single crystal ingot 200 gains in nitrogen concentration toward the lower side and the outer peripheral part of crystal has a low V/G as compared with the central part as shown in FIG. 3b. 
The addition of nitrogen to the CZ-silicon single crystal is implemented by using a nitrogen-doped melt. It is believed that the ratio (segregation coefficient) of nitrogen drawn into the crystal from the melt while the melt is solidifying is extremely small. As a result, the greater part of the nitrogen in the melt remains behind in the melt and the nitrogen concentration in the melt accordingly increases as the growth of crystal proceeds. In the lower part of the crystal, nitrogen concentration is consequently heightened. Although the average temperature gradient G [° C./mm] from the melting point to 1350° C. in the direction of the axis of crystal growth depends on the cooling capacity for the crystal, the value of G is larger in the outer peripheral part of crystal because the outer peripheral part of crystal is generally cooled easily. As a result, the value of V/G is lower in the outer peripheral part of crystal.
The defect region of the nitrogen-doped silicon single crystal of this nature can be described by having the range of growth condition of one silicon single crystal ingot overlap the two-dimensional defect region map using nitrogen concentration and V/G as two axes. In the crystal having the growth condition region as illustrated in FIG. 3, for example, the V region tends to occur in the central portion of crystal and the OSF region in the outer peripheral part of crystal. The void region tends to expand throughout the entire surface of the base plate when the range of nitrogen concentration is fixed and the value of V/G is increased. The void region contracts toward the center of the base plate and the I region tends to expand throughout the entire surface of the base plate when the value of V/G is decreased. When the value of V/G is fixed and the nitrogen concentration is heightened, the OSF region is generated from the outer peripheral part and tends to expand throughout the entire surface of the base plate.
As a result of a detailed study pursued concerning the relation between the epi-layer defect generated in the epitaxial wafer using a nitrogen-doped base plate (substrate), including the fault regions mentioned above and the nitrogen concentration and the V/G rate, it has been found that the defect regions subsequent to the epi-deposition assume such an appearance as shown in FIGS. 4a and b. Here, FIG. 4a is a graph showing the relation of the nitrogen concentration and the V/G rate and FIG. 4b shows the status of occurrence of N-SF and E-pit in the in-plane part of a base plate for several different growth condition regions. In FIGS. 4a and b, the growth condition region 1 has nitrogen concentration of 5×1013 to 1×1014 atoms/cm3 and V/G (relative value) of 1.1 to 2.0, the growth condition region 2 has nitrogen concentration of 1×1014 to 5×1014 atoms/cm3 and V/G (relative value) of 1.1 to 2.0, the growth condition region 3 has nitrogen concentration of 5×1014 to 2×1015 atoms/cm3 and V/G (relative value) of 1.1 to 2.0, and the growth condition region 4 has nitrogen concentration of 1×1014 to 5×1014 atoms/cm3 and V/G (relative value) of 1.3 to 2.0.
The normalized values of V/G shown herein are based on a unity value (1.0) that is the value of V/G at the position at which an OS occurs in a base plate when a crystal with no nitrogen dopant is pulled.
The N-SF and the E-pit, which are epi-layer defects, appear at a position corresponding to the OSF region in a substrate prior to the epi-deposition.
In the growth condition region 1, no epi defect occurs. In the growth condition region 2, N-SF occurs on the outer peripheral side only on the bottom side of a crystal. In the growth condition region 3, N-SF or E-pit occurs on the outer peripheral side from the top side through the bottom side of a crystal.
For the purpose of producing a nitrogen-doped base plate that tends toward no occurrence of crystal fault, like N-SF or E-pit, in an epi-layer 102, it is desirable to control crystal growth so as to exclude the OSF region of a base plate from the outer peripheral part of crystal. As a way of realizing a nitrogen-doped base plate excluding the OSF region mentioned above, a method of lowering the nitrogen concentration or a method of heightening the minimum value of V/G (namely the V/G on the crystal edge side) as shown in the growth condition region 4 may be desirable.
When the nitrogen concentration is lowered as in the growth condition region 1, however, since the density of oxygen precipitation is degraded, the ability of gettering is deteriorated inevitably in spite of prevention of the epi-layer defect.
In Patent Document 2 and Patent Document 3, a method for eliminating an OSF region from a base plate and avoiding an epi-layer defect by controlling crystal growth conditions is disclosed (corresponding to growth condition region 4).
Since the ranges of the optimum nitrogen concentration and pulling conditions necessary for the prevention of epi-layer fault are extremely narrow, and particularly the V/G is greatly varied according to the construction of a pulling oven or furnace and the conditions of the silicon melt, assuring repeatable quality is difficult. Further, this method cannot be applied to a crystal of such a large diameter as 300 mm, for example. This is because in a crystal having a large diameter, the cooling capacity for the crystal deteriorates and the V/G cannot be sufficiently increased.
While increasing the density of the oxygen precipitate subsequent to the epi-deposition calls for an increased nitrogen concentration, the prevention of the epi-layer defect becomes practically impossible when the nitrogen concentration exceeds a certain upper limit. As a result, various users' specifications for the densities of oxygen precipitate cannot be adequately handled because the controllable density of oxygen precipitate ultimately reaches an upper limit.
As a way of avoiding this problem, a method that consists in doping carbon in addition to nitrogen as disclosed in Patent Document 4 is believed to be effective. This is because the crystal defect (presumed to constitute a cause for N-SF and E-pit) formed in the base plate in consequence of the doping of nitrogen is rendered harmless by the simultaneous doping of carbon. The method for avoiding the epi-layer defect by doping carbon in addition to nitrogen as described above (by adding carbon in a ratio of not less than 1×1016 atoms/cm3 to the growth condition region 2 or 3) is capable of preventing the epi-layer defect. This is accomplished without resorting to a measure to increase the lower limit of the V/G value by controlling the crystal growth condition and consequently enabling stable supply of such a large-diameter crystal as avoids forming an epi-layer defect.
In recent years, the trend of increasing the diameter of an epitaxial wafer from 200 mm to 300 mm has given rise to new demands concerning the property of oxygen precipitation.
(1) A demand for uniformly controlling the density of oxygen precipitate subsequent to a heat treatment in the radial direction in the base plate. The reason for this demand is that when the density of oxygen precipitate decreases in the radial direction in the base plate, the base plate suffers deficiency in the gettering ability in the areas of decreased density, which induces a decline in process yield. It has been found that in the case of an epitaxial wafer using a nitrogen-doped crystal, the radial direction distribution of oxygen precipitate closely corresponds to the defect region of the base plate. This fact means that the control of the defect region in the radial direction of the crystal becomes difficult. Also, the radial direction distribution of the density of oxygen precipitate becomes non-uniform because the cooling speed during the growth of crystal differs outside and inside the crystal when the crystal diameter becomes as large as 300 mm.
(2) A demand for increased density of oxygen precipitate. The reason for this demand is that a highly integrated MOS device produced from a large-diameter epitaxial wafer (e.g., measuring 300 mm in diameter) tends to lower the temperature of the heat treatment in device processing (the highest temperature not higher than 1100° C.) as compared with a highly integrated MOS device produced from an epitaxial wafer measuring not more than 200 mm in diameter. This fact means that the heavy metals incorporated in the course of device processing are less likely to be sufficiently diffused and to be absorbed by the oxygen precipitate in the radial direction in the base plate. For the purpose of securing the gettering ability sufficiently subsequent to such a low-temperature and short-duration heat treatment as mentioned above, it is desired that the density of the oxygen precipitate be further increased. As a result of a detailed study, it has been ascertained empirically that when the density of oxygen precipitate in an epitaxial wafer is measured along the radius of the wafer, the decline in the process yield in the radial direction in the base plate due to insufficient gettering becomes conspicuous as the radial distribution variation of the density of oxygen precipitate expressed by the following formula exceeds 0.5. Specifically, the radial distribution variation is found in accordance with the following formula.Radial distribution variation of the density of oxygen precipitate=(Maximum density of oxygen precipitate−minimum density of oxygen precipitate)/Maximum density of oxygen precipitate
When an epitaxial wafer using a nitrogen-doped base plate or a nitrogen- and carbon-doped base plate as a substrate was subjected to a detail study concerning the radial direction distribution of oxygen precipitate subsequent to a heat treatment, it was found that a portion in which the density of oxygen precipitate dropped as compared with the environment thereof was present in the wafer. In the case of the growth condition region 2 or 3 shown in FIG. 4, the radial distribution variation of the density of oxygen precipitate eventually exceeded 0.5 because of the presence of the portion in which the degree of oxygen precipitate dropped as compared with the environment thereof in addition to the formation of N-SF or E-pit.
The method that employs simultaneous doping of nitrogen plus carbon as disclosed in Patent Document 4 is effective in reducing epi-layer faults. Regarding the dispersion of the radial direction distribution of oxygen precipitate, however, the simultaneous doping of carbon has no effect. To be specific, when carbon is doped in a ratio of not less than 1×1016 atoms/cm3 to a crystal in the growth condition region 2 or 3 shown in FIG. 4, the radial distribution variation of the density of oxygen precipitate inevitably exceeds 0.5 because of the presence of a portion in which the density of oxygen precipitate dropped as compared with the environment thereof in spite of the success achieved in preventing the epi-layer defect.
Methods for producing an epitaxial wafer enabled to achieve radial direction uniformity of the density of oxygen precipitate have been known (refer, for example, to Patent Document 6). The technical points of these methods consist in limiting the OSF region at the outside of crystal or contracting it toward the interior of crystal. An effort to limit the OSF region at the outside of crystal corresponds to the growth condition region 1 shown in FIG. 4 and is incapable of gaining in the density of oxygen precipitate sufficiently as already described. An effort to contract the OSF region toward the interior of crystal corresponds to using the I region shown in FIG. 4. The I region causes the density of oxygen precipitate to decrease and the dislocation loop present in the I region gets transferred to an epi-layer and eventually causes an epi-layer defect to form.
Owing to the foregoing state of affairs, it has been difficult to manufacture by the existing technique such an epitaxial wafer as exhibits a high density of oxygen precipitate and allows the density to be uniformly distributed in the radial direction part of a base plate without entailing formation of an epi-layer defect.
[Patent Document 1] JP-A H10-50715
[Patent Document 2] JP-A 2001-106594
[Patent Document 3] JP-A 2002-154891
[Patent Document 4] JP-A 2002-201091
[Patent Document 5] JP-A 2000-331933
[Patent Document 6] JP-A 2003-218120
[Non-Patent Document 1] V. V. Voronkov, K. Crystal Growth, 59 (1982) 625
[Non-Patent Document 2] M. Iida, W. Kusai, M. Tamatsuka, E. Iino, M. Kimura, and S. Muraoka, Defect in Silicon, ed. T. Abe, W. M. Bullisetal (ECS., Pennington N.J., 1999) 499